An STI is known as a technique for electrically isolating a device formed on a silicon substrate. The STI process involves the steps of etching a silicon substrate through a mask of, for example, a silicon nitride film to form a trench therein; filling the trench with an oxide film such as SiO2; and finally planarizing the substrate by chemical mechanical polishing (CMP) by way of using the mask (silicon nitride film) as a stopper (see, for example, U.S. Pat. No. 6,844,265).
The silicon trench etching process of the STI includes a break-through (BT) step of performing an etching to remove a native oxide film formed on the surface of a silicon layer after patterning by using, for example, the silicon nitride film as a mask and a main step of performing an etching on the silicon layer from which the native oxide film is removed, to thereby form a trench in the silicon layer (see, for example, Japanese Patent Laid-open Publication No. H11-214356).
Moreover, as a technique related to the STI, there is proposed a method for forming in advance an end point detection layer in the mask to facilitate the detection of an end point of a CMP process, to thereby control a thickness of a device isolation film (see, for example, Japanese Patent Laid-open Publication No. 2003-45956).
Meanwhile, U.S. Patent Publication No. 2004/0191932 discloses a method for performing a plasma etching on a silicon oxide film by using a resist film as a mask, while monitoring the thickness of the resist film by employing an optical method, to thereby prevent a reduction in the thickness of the resist film.
Since the native oxide film formed on the surface of the silicon substrate after the patterning can be removed in a relatively short period of time ranging from about 5 to 10 seconds, the end point of the etching in the BT step is conventionally set based on a lapse of predetermined time. Though the BT step is performed under a condition in which even a hard mask such as a silicon nitride film can be easily etched, if the end point of the BT step is controlled indiscriminately based only on time, the etching will be terminated at a predetermined time point regardless of the residual film thickness of the mask, resulting in non-uniformity in the film thickness of the mask after the BT step. That is, since in general there is variation in the film thickness of the mask present even prior to performing the etching, the film thickness of the mask may still remain non-uniform after the BT step is performed. In the main step following the BT step, the etching rate of silicon is high, whereas etching is performed very slowly on an oxide film or a nitride film. Therefore, a surface polishing is to be conducted during the subsequent CMP process under a condition in which the film thickness of the mask is not uniform.
In the CMP process, since polishing is typically set to be finished upon the exposure of the mask layer such as the silicon nitride film, the residual amount of the buried oxide film may become irregular if the film thickness of the mask layer is non-uniform. Moreover, if the film thickness of the mask is non-uniform, some part of the mask may be left as a residue when the mask is removed after the CMP process by, for example, wet etching. Though it is technically possible to prepare an end point detection layer in the mask to avoid these problems, as disclosed in Japanese Patent Publication No. 2003-45956 cited above, it is not considered practical since it will increase the number of processes required.